Serum concentrations of all-trans and 13-cis retinoic acid and retinol are closely correlated

The origin and role of all-trans and 13-cis retinoic acid in serum is unknown. Certain tissues are known to oxidize retinol via retinaldehyde to retinoic acid. Some p450 isoenzymes are down-regulated in inflammation and as retinoic acid is catabolized by members of this enzyme family, we hypothesized that the concentration of all-trans and 13-cis retinoic acids might be increased in inflammation. Serum retinol, on the other hand, decreases in inflammation. We, therefore, measured the serum concentration of the retinoic acids, retinol and the other components of the retinol transport system, the retinol-binding protein, and transthyretin in patients with inflammation. The degree of inflammation was judged from the serum level of C-reactive protein. We found that retinol, all-trans and 13-cis retinoic acid concentrations in serum, were all decreased in inflammation and that they were significantly and positively correlated to each other. The serum concentrations of 13-cis retinoic acid retinol, retinol-binding protein, and transthyretin concentrations, but not ail-trans retinoic acid were all significantly and negatively correlated to the concentration of C-reactive protein. Retinol concentration in the serum may be one of the determinants of the serum retinoic acid concentration. We speculate that the decrease in serum retinol and retinoic acid concentration, which occurs in inflammation may create an ''acute vitamin A deficiency'', which may be a factor contributing to the excess mortality associated with measles in children which marginal vitamin A deficiency and in patients with AIDS.